-LRB- CNN -RRB- -- Mexican authorities on Saturday arrested four men in connection with last week 's shooting death of a U.S. Border Patrol agent in San Diego County , California , Mexico 's state-run news agency Notimex reported .

U.S. Border Patrol Agent Robert Rosas was fatally shot Thursday night in California , U.S. authorities said .

Mexican federal police identified the men as human smugglers , and said they were in the act of transporting 21 immigrants when they were detained in the northwest state of Baja California , Notimex said .

At a news conference , federal police identified two of the suspects as brothers Jose Eugenio Quintero Ruiz , 49 , and Jose Evodio Quintero Ruiz , 43 . The other two arrestees were taxi drivers Antonio Badallares Zepeda , 57 and Jose Alfredo Camacho Penuela , 34 , Notimex reported .

Border Patrol Agent Robert Rosas was shot and killed Thursday night while responding to a potential incursion into the United States in the Campo area in San Diego County , U.S. authorities said .

The Mexican federal police did not offer specific evidence of the suspects ' role in the killing , but said intelligence reports indicated the group was responsible for kidnappings , rapes and murders of several people who tried to cross to the United States , Notimex said . The men were wanted by American authorities , police said .

Notimex said that during his interrogation , Jose Eugenio Quintero told investigators the shooter was Ernesto Parra Valenzuela , a man arrested the day before by local police in Tecate , Mexico .

Rosas , who is survived by his wife and two young children , had been a border agent for three years .

Rosas was the ninth Border Patrol agent to be killed while on duty since 2006 , according to the agency 's Web site .

Two agents died in a vehicle wreck in 2006 , and four died in 2007 , including two who died in vehicle wrecks , a third who drowned and a fourth who suffered a heart attack while pursuing undocumented immigrants .

Two agents died on duty last year , the Border Patrol said . One died in a single-vehicle wreck ; another was struck and killed by a vehicle driven by a suspected smuggler , according to the agency 's Web site .

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Mexican news agency : Four arrested in connection with U.S. agent 's death

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U.S. Border Patrol agent Robert Rosas gunned down Thursday in California

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Mexican police say suspects are smugglers of humans

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News agency : Suspect says fifth man -- arrested earlier -- was shooter